Two test crosses are made with plants that are heterozygous for two genes…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Two test crosses are made with plants that are heterozygous for two genes. The offspring come out like this.
- Cross 1: AaBb, aabb, Aabb and aaBb.
- Cross 2: AaBb, aabb, Aabb and aaBb.
- In which cross are the genes linked, and how can you tell?
- What is the recombination frequency in cross 2?
- What would you expect in cross 1 if the genes were on different chromosomes?
Need a reminder? The key ideaShow
An X-linked recessive allele (h) shows in every man who has it, and in a woman only if she has two copies. A carrier woman is . Carrier mother × normal father: one quarter of all the children are haemophilic sons. Haemophilic father × normal mother: all daughters are carriers and all sons are normal.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. Cross 1: . Cross 2: .
Plan. Four equal classes mean independent assortment. Two big and two small classes mean linkage.
Step 1. (a) In cross 2, the two parental classes (AaBb and aabb) are much bigger than the two recombinant classes. The genes are linked. In cross 1, the four classes are nearly equal.
Step 2. (b) The recombinants are out of : .
Step 3. (c) In cross 1 the genes appear to be on different chromosomes, or very far apart. A dihybrid test cross then gives , and the four classes in cross 1 match that.
Answer: (a) cross 2; (b) ; (c) four nearly equal classes, as seen.
Check. In cross 1 the recombinants (Aabb and aaBb) are out of , which is , the largest value possible.
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